The present invention concerns pneumatic tires for motor vehicles and more specifically it relates to their beads, namely to the annular portions of the tire carcass which have the task of anchoring the tire on the wheel rim upon which it is mounted. More precisely the invention refers to the reinforcing annular elements inserted in the beads, which are commonly defined as bead cores and an assembled core ready to be placed in a tire, known as a "bead core package" or ring.
The various types of wheel rims for tires are so widely known that there is no need of describing them in the present specification; it is sufficient to point out that they are provided at their tire bead mounting positions with two or more less conical coaxial surfaces, representing the supporting base for the tire beads; these surfaces terminate in a flange, radially projecting outward, which is intended to support the axially outer surface of the bead and against which the latter abuts by virtue of the tire inflation pressure.
Proper positioning of the bead into its seat is ensured by the conical shape of the bead seat in cooperation with the reinforcing metal bead core. This core is circumferentially inextensible having therefore a constant diameter, which is contained in the tire bead; the tight fit of the tire bead on the conically tapered bead seat, created by the axial thrust which is exerted on the bead side, from the inside out, on account of the tire inflation pressure, ensures the anchorage of the tire on the rim during use and in tubeless tires, also the air tightness between tire and rim to prevent a progressive deflation of the tire.
It is known that a tubeless tire, namely a tire not provided with inner tube, must be assembled on the so-called grooved rims, i.e. one-piece rims in which the bead are considerably conical (15.degree. with respect to the axis of rotation of the rim); also, it can be understood that, to assemble the tire on the rim and to remove it therefrom, the tire bead must step over the rim flange which has a diameter greater than the radially inner diameter of the core. To carry out this operation, the core must be flexible, that is capable of deformation and in particular it must take an elliptical configuration (ovalization) which enables it to step over the rim flange in consequence of the diameter increase which takes place at the greater axis of the ellipse.
The solution of this problem proves particularly difficult in respect of large-size single-core tires, owing to the relevant geometrical size of the cores. In fact, it is evident that cores made of rubberized wires, like those at present used for large-size tires, after the tire curing becomes rigid and extremely compact structures, which are therefore scarcely flexible. For tires of this kind, provision was therefore made of cores composed of bare wires, able to displace circumferentially with respect to one another and consequently to allow the deformation and in particular the ovalization of the core even in the cured tire. These cores are built up in two different versions, namely one having a circular section, commonly known as "cord", made of a single wire of circular section helically wound up in several coils around a central element, and one of quadrangular straight section, obtained by means of a metal strap having quadrangular straight section with at least two opposite sides parallel to each other.
However, these cores show some disadvantageous characteristics both as regards their construction and as regards their employment.
As to the cores of the "cord" type, their section proves substantially inadequate in view of their use in tubeless tires, since--besides having a certain circumferential elongation it has little resistant to torsional stresses and consequently is unable to ensure the anchorage of the tire bead on the rim during operation, which is a substantial condition necessary to maintain the regular inflation pressure and to safeguard the structural integrity of the tire.
The "metal strap" type cores are made by winding up on an appropriate drum, in several radially superimposed turns, a plurality of straps arranged side-by-side, so that the final core is constituted by a plurality of coils in side-by-side relationship, but not linked to one another. Further, to impart to the base of the finished core the inclination of 15.degree. on the horizontal plane (core axis), which is strictly necessary to use the core in tubeless tires, the metal strap is appropriately pre-shaped before being wound up on the drum and is then wound up on it with its side facing the drum already inclined by 15.degree..
Clearly, this production system involves a more complicate machinery and increases costs: further, the obtained core shows by itself an insufficient resistance to the stresses exerted onto it during the tire moulding and in operation, so that the necessary compactness of its section, and consequently its stability and torsional strength can be obtained and maintained only if the core body, that is the straps package, is kept closed and compact by a set of metal strips or check elements wound up around the package and circumferentially arranged in spaced relationship along the circumferential development of the core in order to prevent any relative displacement between the straps both in axial and in radial direction. Also this expedient involves obviously additional costs and complications.
Besides this, in order to achieve a greater compactness of the core to ensure the stability of its shape during the tire moulding and to maintain the integrity of the bead in course of time, the metal core was embedded in a kern of very hard elastomeric material, having a substantially circular section and covered in turn with a sheet of elastomeric material of still higher hardness so as to create a closed tubular envelope within which is embedded the metal core; in fact, only in this way was it possible to prevent the tears in the elastomer material and the detachments of said material from the core metal surface which--in other cases--unavoidably place during the tire operation on account of the torsional stresses acting on the bead, an example of such a construction is shown in U.S. Pat. No. 3,949,800 to Lejeune.